For the UDF mechanism to work, functions must be written in C or
C++ and your operating system must support dynamic loading.
MySQL source distributions include a file
sql/udf_example.c that defines five UDF
functions. Consult this file to see how UDF calling conventions
work. The include/mysql_com.h header file
defines UDF-related symbols and data structures, although you
need not include this header file directly; it is included by
mysql.h.

A UDF contains code that becomes part of the running server, so
when you write a UDF, you are bound by any and all constraints
that apply to writing server code. For example, you may have
problems if you attempt to use functions from the
libstdc++ library. These constraints may
change in future versions of the server, so it is possible that
server upgrades will require revisions to UDFs that were
originally written for older servers. For information about
these constraints, see
Section 2.9.4, “MySQL Source-Configuration Options”, and
Section 2.9.5, “Dealing with Problems Compiling MySQL”.

To be able to use UDFs, you must link mysqld
dynamically. If you want to use a UDF that needs to access
symbols from mysqld (for example, the
metaphone function in
sql/udf_example.c uses
default_charset_info), you must link the
program with -rdynamic (see man
dlopen).

For each function that you want to use in SQL statements, you
should define corresponding C (or C++) functions. In the
following discussion, the name “xxx” is used for an
example function name. To distinguish between SQL and C/C++
usage, XXX() (uppercase) indicates an SQL
function call, and xxx() (lowercase)
indicates a C/C++ function call.

Note

When using C++ you can encapsulate your C functions within:

extern "C" { ... }

This ensures that your C++ function names remain readable in
the completed UDF.

The following list describes the C/C++ functions that you write
to implement the interface for a function named
XXX(). The main function,
xxx(), is required. In addition, a UDF
requires at least one of the other functions described here, for
reasons discussed in Section 24.4.2.6, “UDF Security Precautions”.

xxx()

The main function. This is where the function result is
computed. The correspondence between the SQL function data
type and the return type of your C/C++ function is shown
here.

It is also possible to declare a
DECIMAL function, but
currently the value is returned as a string, so you should
write the UDF as though it were a STRING
function. ROW functions are not
implemented.

xxx_init()

The initialization function for xxx(). If
present, it can be used for the following purposes:

To check the number of arguments to
XXX().

To verify that the arguments are of a required type or,
alternatively, to tell MySQL to coerce arguments to the
required types when the main function is called.

To allocate any memory required by the main function.

To specify the maximum length of the result.

To specify (for REAL
functions) the maximum number of decimal places in the
result.

To specify whether the result can be
NULL.

xxx_deinit()

The deinitialization function for xxx().
If present, it should deallocate any memory allocated by the
initialization function.

When an SQL statement invokes XXX(), MySQL
calls the initialization function xxx_init()
to let it perform any required setup, such as argument checking
or memory allocation. If xxx_init() returns
an error, MySQL aborts the SQL statement with an error message
and does not call the main or deinitialization functions.
Otherwise, MySQL calls the main function
xxx() once for each row. After all rows have
been processed, MySQL calls the deinitialization function
xxx_deinit() so that it can perform any
required cleanup.

For aggregate functions that work like
SUM(), you must also provide the
following functions:

xxx_clear()

Reset the current aggregate value but do not insert the
argument as the initial aggregate value for a new group.

xxx_add()

Add the argument to the current aggregate value.

MySQL handles aggregate UDFs as follows:

Call xxx_init() to let the aggregate
function allocate any memory it needs for storing results.

Sort the table according to the GROUP BY
expression.

Call xxx_clear() for the first row in
each new group.

Call xxx_add() for each row that belongs
in the same group.

Call xxx() to get the result for the
aggregate when the group changes or after the last row has
been processed.

Repeat steps 3 to 5 until all rows has been processed

Call xxx_deinit() to let the UDF free any
memory it has allocated.

All functions must be thread-safe. This includes not just the
main function, but the initialization and deinitialization
functions as well, and also the additional functions required by
aggregate functions. A consequence of this requirement is that
you are not permitted to allocate any global or static variables
that change! If you need memory, you should allocate it in
xxx_init() and free it in
xxx_deinit().

I was wondering if there is a way to change allocated memory for executing UDF in MySQL ?

Well, here is my problem :I'm trying to call java functions inside my C - UDF via JNI. After compilation everything works well. The dynamic library has been copied in the mysql plugin dir, and I have added the function into MySQL.

When I run the query, calling the UDF function (which calls my java library vi JNI), the first execution is fine, but for the second one I got a MySQL Server CRASH!

So it always run once, and crashes the second time.... Maybe a memory problem since UDF run in-process with MySQL daemon ?